(1) Field of the Invention
The present invention concerns selective gas permeation membranes and a method of manufacturing them. More specifically, it relates to selective gas permeation membranes which are excellent, particularly, in the permeability and separatability of oxygen molecules, capable of obtaining oxygen-enriched gases or oxygen-reduced gases, such as nitrogen-enriched gases efficiently by removing oxygen from oxygen-containing gases such as air and capable of being used suitably as various gas separation membranes for use in combustion facilities, foodstuff industries, medical equipments and waste disposing facilities, as well as to a method of manufacturing such gas permeation membranes with practical advantage.
(2) Description of the Related Art
As the gas separation method, various methods have long been employed such as cooling-condensation-evaporation, adsorption separation, solvent extraction separation, etc.
By the way, these methods employed generally require a great amount of energy and complicated facilities. In view of the above, gas separation, by using membranes made of polymeric material, have been attempted generally intending to reduce the consumption of energy.
Gas separation methods using such polymeric membranes have been known, for example:
(1) Use of polyorganosiloxane of excellent selective gas permeability, particularly, selective permeability of oxygen molecules in the form of a copolymer or a blend with other polymeric material, for example, polycarbonate, polyurethane, polystyrene or polyphenylene oxide with an aim of improving the strength, etc. of the membranes (refer to Japanese Patent Laid-Open Nos. Sho 48-64199, 58-163,403, 58-14926, etc.),
(2) Use of porous polymeric material for the membranes, and
(3) Use of composite membranes prepared by applying a thin film to the surface of a porous support (for instance refer to Japanese Patent Publication No. Sho 59-3201).
However, the membranes in (1) and (3) above are not sufficient in the gas permeability, whereas the membranes in (2) above are insufficient for the selectivity.
Further, although a method of impregnating pores of a porous polymeric material with polyorganosiloxane has also been attempted, it involves a problem that only low gas permeability has been achieved by such means, failing to obtain sufficient practical performance.
On the other hand, there has also been proposed gas separating composite membranes in which thin films of silver, or like other metal are formed at the surface or on the inside of a polymeric membrane (Japanese Patent Application Laid-Open No. Sho 58-8510).
However, in a case where air is caused to permeate through the composite membrane, although the separation coefficient for O.sub.2 /N.sub.2 is increased to 3.0 as compared with that of 1.0 in a case where no metal thin films are formed, the permeation coefficient is rather reduced to as low as 7.0.times.10.sup.-9 cc-cm/cm.sup.2.sec.cmHg as compared with that of 1.1.times.10.sup.-7 cc-cm/cm.sup.2.sec.cmHg in a case where no metal thin films are formed. Accordingly, this is not quite satisfactory in view of the permeation coefficient.
It has further been reported for the separation of oxygen-nitrogen the use of Nafion-silver composite membranes in which silver is deposited by way of sulfone groups on Nafion membranes having such groups (refer to "Journal of Membrane Science", vol. 31, -227 (1987)).
However, in the Nafion-silver composite membranes, although the O.sub.2 /N.sub.2 separation coefficient is remarkably increased to 11.0, as compared with that of 2.0 in a case where silver is not deposited, the permeation coefficient is decreased, although little, to 1.0.times.10.sup.-9 cc-cm/cm.sup.2.sec.cmHg as compared with that of 1.5.times.10.sup.-9 cc-cm/cm.sup.2.sec.cmHg in a case where silver is not deposited and the value for the permeation coefficient is not quite satisfactory.